This invention relates to electronic circuits such as noise filters and sample and hold circuits that are switched into and out of radars, analog/digital converters and other electronic systems. In particlar, the invention relates to fast operating switchable operational amplifier driven circuits of that type.
In many radar and ECCM applications, it is required to either quickly sample and hold for a long period of time a high frequency analog signal, or to quickly switch into the path of a high frequency analog signal a low pass filter with a very long time constant. Such requirements are particularly found in the design of high speed A/D converters, where sample and hold circuits are used, and in the design of low noise STALO's, where switchable low pass filters are used.
Switchable noise filters generally comprise a driving operational amplifier the output of which is connected to its output through a resistor. A capacitor is connected between the output and ground. Switching to determine low frequency break is accomplished by a field effect transistor switch that parallels the resistor. The sample and hold circuit is similar except that the resistor is eliminated and the field effect transistor switch is inserted in and switches the operational amplifier output line directly. In both circuits for a given capacitor value the switching time is limited and determined by the ON resistance of the field effect transistor switch.
The present invention is directed toward improving the switching time in such circuits by employing an operational amplifier having two alternative feedback loops and utilizing the excess feedback loop gain resulting therefrom to reduce effective resistance in the capacitor charging circuit.